Channeling scattering yield

One of the most fascinating applications of channeling RBS is the study of lattice locations of impurity atoms. By measuring the angular dependence of the back-scattering yield of the impurity and host atoms around three independent channeling axes it is possible to calculate the position of the impurity. Details can be found elsewhere [3.122]. 

Key topics covered in the review are the analysis of the wavepacket in the exit channel to yield product quantum state distributions, photofragmentation T matrix elements, state-to-state S matrices, and the real wavepacket method, which we have applied only to reactive scattering calculations. 

The reduction in scattering yield associated with channeling can be applied to determine the lattice site position of impurity atoms and defects in the crystal (Fig. 8.3). An impurity on a lattice site has a reduction in scattering yield equal to that of the bulk crystal interstitial impurities or atoms located more than 0.1 A from a lattice site are exposed to the flux of channeled ions. Consequently, the backscattering yield from such nonsubstitutional atoms does not exhibit the same decrease as that of the host crystal. 

The effects of lattice disorder defects and crystal imperfection on channeling are used to analyze ion-implanted samples. Host atoms displaced from their lattice sites can interact with the channeled beam, leading to an increase in the scattering yield. 

Based on this physical view of the reaction dynamics, a very broad class of models can be constructed that yield qualitatively similar oscillations of the reaction probabilities. As shown in Fig. 40(b), a model based on Eckart barriers and constant non-adiabatic coupling to mimic H + D2, yields out-of-phase oscillations in Pr(0,0 — 0,j E) analogous to those observed in the full quantum scattering calculation. Note, however, that if the recoupling in the exit-channel is omitted (as shown in Fig. 40(b) with dashed lines) then oscillations disappear and Pr exhibits simple steps at the QBS energies. As the occurrence of the oscillation is quite insensitive to the details of the model, the interference of pathways through the network of QBS seems to provide a robust mechanism for the oscillating reaction probabilities. 

Fig. 13. Measured channeling dips in the yield of elastically scattered 670 keV protons from the Si lattice (O) and the yield of the (p, a) nuclear reaction with UB atoms (A). The difference in the angular widths of the two dips is due to displacements of the boron atoms in B—H complexes from substitutional sites. From Marwick et al. (1987)...

I would like to ask Prof. J. Troe whether he could discuss some typical situations where the SAC approximation may fail. For example, consider the F + HBr — FHBr — HF(u) + Br reaction with energy E just above the potential barrier V41. In this situation, the adiabatic channels in the transition state ( ) should be populated only in the vibrational ground state, and they should, therefore, yield products HF(u = 0) + Br, according to the assumption of adiabatic channels. This is in contrast with population inversion in the experimental results that is, the preferred product channels are HF(i/) + Br, where v = 3, 4 [1] see also the quantum scattering model simulations in Ref. [2]. The fact that dynamics cannot be rigorously adiabatic (as in the most literal interpretation of SAC) has been discussed by Green et al. [3], and the most recent results (for the case of ketene) are in Ref. 4. 

---